MAY FLIES AND MIDGES OF NEW YORK 85 



the thorax as a whole is called the sternum or pectus. The inter- 

 mediate legs are attached to the hind part of the mesosternum by 

 oval sockets. The metathorax is much smaller than the meso- 

 thorax. Its dorsal surface, called the metanotum, has already 

 been mentioned. On the side is the posterior thoracic spiracle, 

 and above it is the haltere (balancer or poiser) the rudimentary 

 hind wing, a slender organ with a dilated head. The ventral sur- 

 face of the metathorax is short and narrow and is largely occupied 

 by the insertion of the hind legs. 



The abdomen is composed of nine segments more or less 

 closely fused together. In the male especially it is long and 

 slender and terminates with the genitalia. The genitalia varies 

 greatly with the different genera (pis. 32, 33). The anus opens 

 on the dorsal surface of the ninth segment. The under surface of 

 abdomen is sometimes called the venter. 



The three pairs of legs are long and slender, especially so in 

 Chiron omus, are attached to the prothorax, mesothorax 

 and metathorax, and are called respectively the front, middle and 

 hind pairs. The older writers who used the Latin terminology 

 spoke of the forelegs as pedes antici, the middle legs as pedes 

 medii, and the hind legs, pedes postici. When they spoke of the 

 fore and middle legs together they called them pedes anteriores. 

 Some writers still use the term anterior legs for fore and middle 

 pairs, and posterior legs for the middle and hind pairs. The 

 coxa is the part attaching the leg proper to the thorax; while 

 the trochanter is the short, small, ring-like portion between the 

 femur and coxa. The femur or thigh is the stoutest portion of 

 the leg; the tibia is the next part succeeding the femur. The tarsus 

 is the distal division of the leg and is composed of five joints, of 

 which the first, that next the tibia, is called the metatarsus. 

 The ungues or claws are two booklets on the underside of the 

 last tarsal joint. In most genera these are simple, but a few 

 liave uniserrate or bifid claws. The pulvilli, two pad-like fleshy 

 cushions attached to the last joint of the tarsus below the claws, 

 are often present. The empodium is a median appendag'e between 

 the claws, and is usually present also. The usual shape in this 

 family is that of a sickle-shaped process, pectinate on the convex 

 side. 



